The present invention relates to a switching scheme in a three phase alternating current circuit. More specifically, the present invention is directed to rearranging a three phase circuit into delta and star connected circuits with minimal switches to provide two ratings to the circuit.
Typically three phase circuits are connected in delta or star. It is well-known that if the same three phase circuit is arranged to operate in either delta or star connection, the ratings of the circuit would be different. FIG. 1 shows a conventional and straight-forward switching arrangement that permits operation of a three phase circuit, with impedance Z ohms per phase, in delta or star configuration. When switches designated Sd are closed and switches designated Ss are open, the circuit is connected in delta. If the switches Sd are open and switches Ss are closed, the circuit will be connected in star. If the phase to neutral applied voltage va, vb and vc have an rms value of V volts, the voltage stress vd and the volt-ampere rating Pd in the case of delta connection would be:
vd=/{overscore (3)}V/Z volts per ohmxe2x80x83xe2x80x83(1)
Pd=9V2/Zxe2x80x83xe2x80x83(2)
The voltage stress vs and volt-ampere rating Ps for the star connection would be:
vs=V/Z volts per ohmxe2x80x83xe2x80x83(3)
Ps=3V2/Zxe2x80x83xe2x80x83(4)
The changes in voltage ratings for the delta and star connection would be:                                           v            d                                v            s                          =                  /                      3            _                                              (        5        )                                                      P            d                                P            s                          =        3                            (        6        )            
An arrangement that gives two ratings for the same bank could be useful in certain circumstances. One application would be to increase the reactive power rating of the circuit when there is significant variation in applied voltages. A circuit connected star may be reconnected in delta to offset the reduction in power or reactive power due to drop in applied voltage. Another application would be to tap the temporary overload ratings of capacitor banks to cope with some temporary condition in the power system with or without change in the applied voltages. For example, during a disturbance in the power system, it may be possible to beneficially vary the reactive power output of a capacitor bank since capacitors typically have significant short term overload capability.
The method shown in FIG. 1 when applied to capacitor banks has a number of drawbacks. It needs a large number of switches with relatively high voltage ratings. Also, the changes in the voltage stress on the impedance elements and the volt-ampere ratings between the two connections are too large for many practical applications. A more modest change in the voltage stress comparable to permissible temporary overload ratings of the capacitors, would have better scope for practical application. The method also requires that the bank be temporarily disconnected during the changeover from one connection to the other. This could have objectionable system impact. For example, in switching a capacitor bank with two ratings, Q1 and Q2, Q2 being higher than Q1, the maximum switched block will be Q2, with corresponding system voltage change. Comparing this with two banks with ratings of Q1 and (Q2xe2x88x92Q1), for which the maximum switched block will be only Q1 or (Q2xe2x88x92Q1), this voltage change will be higher, and possibly objectionable. It is the objective of the present invention to overcome these drawbacks.
Large capacitor banks in power systems are made up of series and/or parallel connected capacitor units of lower voltage ratings. Such banks can be easily split into two sections of equal or unequal ratings. Two alternate arrangement for connecting those two capacitor sections, in one case tie two sections in series and in the other case the two sections in parallel, would facilitate having capacitor ban with dual ratings. In the present invention, deltaxe2x80x94star switching different from the one shown in FIG. 1 is applied to capacitor banks to obtain benefits of reduced switching overvoltages, reduction in high voltage switchgear and ability to tap the inherent temporary overvoltage ratings of capacitors.